In processes which consume steam it is common practice to generate the steam in a boiler at a pressure higher than the pressure required by the process, and then to expand this steam through a turbine before injecting it into the process for use and consumption. The turbine performs useful work, typically driving an electrical generator. It is logical to consider that the cost of generating steam at the pressure required by the process is chargeable to the process; on this basis, the work done by the turbine is obtained for the additional cost of operating the boiler at the higher temperature and pressure which are required by the turbine, and this additional cost is normally small when compared with the value of the work done. The turbine is said to be superposed on the process, and superposed turbines are often attractive economically.
For a process which returns little or none of the condensed steam to the boiler, it is necessary to add fresh water to the boiler continuously. For low pressures, this causes no great problem. It is feasible to operate boilers at pressures up to 400 psi with water which has been taken from wells or streams and treated by comparatively inexpensive means. Unfortunately, however, increases in boiler pressure cause major increases in the stringency of the requirement for purity of boiler water. For chemical processes which are supplied with steam at 300 to 600 psi (or higher) from a superposed turbine, the boiler and turbine must operate at substantially higher pressures (typically 1000 to 1500 psi). Techniques are known for treating water sufficiently for satisfying the purity requirements of boilers operating at pressures in this general range. Between 1500 and 2000 psi this becomes even less practical as pressure is increased. For pressures 2000 psi and higher, however, chemical treatment of any large fraction of the quantity of water required by a given boiler is impractical physically in terms of size and quantities of materials as well as economically in terms of costs.
It is commonplace to operate boilers and turbines at sub-critical pressures as high as the general 2500 psi range, and even in the super-critical range of about 3500 psi; but such pressures have been restricted to processes which return substantially all of the steam in the form of high purity condensate for re-injection into the boiler. In such processes, the only necessary water treatment to high purity after the initial purified charge is the small quantity necessary to replace leakages and losses plus, in certain cases, a relatively simple treatment to reduce or eliminate impurities which find their way into the system. For this, the cost of treatment can be tolerated. However, it was infeasible to consume steam of this purity in another process although valuable energy remained in the steam on exit from the turbine.
Continuous processes for the production of gaseous fuel from a solid carbonaceous fuel are known, such as that in U.S. Pat. No. 4,074,981. Such processes require large quantities of steam for injection into the gasifier to serve as a reagent for the production of the gaseous fuel. In addition, steam entering the gasifier should be at a pressure of several hundred pounds per square inch, for example in the range of about 550 psi. Steam for this gasification process is typically produced in a boiler which is continuously fed by a stream of feedwater. However, feedwater purity requirements for the gasification process are considerably lower than those for steam turbine power generation at the same temperatures and pressures.
Previous attempts to efficiently combine coal gasification with steam turbine power generation have focused on utilizing waste heat from the gasification process, e.g. heat from reducing the temperature of product gas, for heating feedwater, for producing steam to drive steam turbines. Although acceptable for obtaining improvement in overall power plant efficiency, such methods have limitations. U.S. Pat. No. 3,873,845, for example, involves a process combining coal gasification with power generation in which waste heat from the gasification process is used to produce steam for power generation. The steam is produced from heat exchangers downstream from the gasification process. The waste heat produces steam at a pressure of 120 atmospheres and a temperature of 520.degree. C. Although this utilization of heat generated by cooling the product gas affords some improvement in overall plant efficiency, it has limitations. For example, contamination of the steam turbine feedwater can occur in heat exchangers used for cooling the product gas. Such contamination can be detrimental to steam turbine elements.
U.S. Pat. No. 4,043,130 relates to a turbine generator cycle for provision of heat to an external heat load. A dual purpose power generation facility combines electric power generation with a brine desalinization process. The power plant of the generator facility includes a series of high pressure and low pressure steam turbines mechanically linked on a common shaft connected to an electrical generator element. Each of the turbines is connected to a heat exchanger where heat is extracted in the form of steam which is transferred to the desalinization process.
In this system, there is a closed loop arrangement for maintaining steam turbine feedwater in isolation from the desalinization feedwater. The system, however, is designed for operation at relatively low temperatures and pressures, where contamination is not a severe problem. Although acceptable for a process which requires steam at relatively low pressures, e.g. in the 25 psi range, the system is not suitable for steam consuming as opposed to heat consuming processes which require steam in the range of 300-600 psi or higher.